Giant magnetoresistance (GMR ) is the effect observed in multi-layer
structures of magnetic and non magnetic alloys, where changes in the
applied magnetic field cause a large change in resistance. The technology
is widely used in sensors employed by pneumatic cylinder manufacturers
for accurate detection of piston position. Now, a new range of GMR
sensors has been introduced for gear teeth sensing and magnetic encoders,
which are resistant to high temperatures and undamaged by large magnetic
fields. Available in the UK from Rhopoint Components, these new sensors
are complemented by a range of support integrated circuits, which
compensate for variations in magnet strength and position.
GMR sensors have significant advantages over Hall Effect and AMR sensors.
In virtually every application they outperform other technologies - often
at a significantly lower installed cost. They have typically fifty to one
hundred times the sensitivity of Hall, and five to ten times the
sensitivity of AMR devices, and are less affected than Hall devices by
variations in temperature. In a ceramic package, a GMR standard
Wheatstone bridge sensor will operate to 220°C, making it suitable for
automotive and other high temperature applications.
In certain applications, such as magnetic encoders and gear teeth
sensing, the user needs to detect not only the variation of a magnetic
field, but also its speed and direction. This led to the development of a
new family of GMR sensors the gear tooth (GT) series, which are designed
specifically for use in industrial speed applications where magnetic
detection of gear teeth and magnetic encoder wheels are required.
GT Sensors are available with both analogue and digital outputs. Single
and double bridge versions are available; the second output being phase
shifted with respect to the first, to provide quadrature for determining
direction. The digital version takes advantage of the high performance
characteristics of GMR sensors to provide a 50% duty cycle output with a
wide tolerance in air gap and temperature variations. The analogue
version provides a sinusoidal output signal when used with a bias magnet
and gear tooth or a magnetic encoder. These sensors feature excellent air
gap performance allowing the design engineer to relax the mechanical
tolerances.
To assist in the development with this new range of sensors, Rhopoint can
provide an evaluation kit, which includes analogue and digital versions
of the sensor product line, plus a stand alone signal processing IC. A
variety of PCB configurations are provided so that the parts can be
tested in different housing and barrel sizes, including M8. Magnets for
biasing are also included, along with application notes on CD-ROM.